1. Field of the Invention
The present invention relates to apparatus for detecting the edges of a pattern formed on an object by scanning the object surface with a laser beam.
2. Related Background Art
The above-mentioned type of edge detection apparatus has been applied, for example, to a line width measuring apparatus of the type in which the line width of a very fine pattern such as an integrated circuit pattern is measured by photoelectrically detecting the edges of the fine pattern.
U.S. Pat. No. 4,112,309 has disclosed apparatus for photoelectrically measuring the coordinates of a pattern such as a pattern formed on a semiconductor wafer placed on a moving stage. The known apparatus comprises a laser, a focusing optical system for focusing the laser light beam to form a laser spot on the wafer, means for oscillating the laser spot on the pattern and a detector for scanning the pattern with the oscillating laser spot and detecting the scattered light from the edge of the scanned pattern. The direction of scanning is changed in accordance with the direction of the edge of the pattern. When the scanning direction is changed, it is necessary to change also the direction of the oscillation of the laser spot. To this end, in the prior art apparatus, there is provided an image rotator in the focusing optical system.
The image rotator must be mounted very precisely so that its rotation axis may be accurately aligned with the optical axis of the focusing optical system. However, it is difficult to completely exclude error in manufacturing or assembling the apparatus. If any error is introduced in the alignment of the axis of the image rotator, some measuring error may be caused by it because the center of the laser spot whose direction of oscillation is changed by the rotation of the image rotator is deviated from the optical axis of the objective lens by the alignment error.
In order to prevent such measuring error, the deviation of the spot center from the optical axis must be corrected every time after the rotation of the image rotator by a certain rotational angle. Therefore, a very large quantity of work and time is required for this correction. This has been one of the most important problems involved in the prior art apparatus.